1. Field of the Invention
The present invention relates to a planar lightwave circuit used for performing optical communications of a WDM (Wavelength Division Multiplexing) transmission system, for example, and to a tunable laser device having the planar lightwave circuit. Hereinafter, the “planar lightwave circuit” is simply referred to as “PLC”, a “semiconductor optical amplifier” as “SOA”, and a “Mach-Zehnder interferometer” as “MZI”, respectively.
2. Description of the Related Art
In the optical communications, there has been active adoption of a WDM transmission system which achieves high-speed optical communications through conducting transmissions with a single optical fiber by multiplexing a plurality of optical signals of different wavelengths, in order to enable efficient utilization of the optical fiber. Further, there has also been a spread of a high-density WDM (DWDM: Dense WDM) transmission system which enables high-speed transmissions by multiplexing optical signals of several tens of different wavelengths.
Further, ROADM (Reconfigurable Optical Add/Drop Multiplexer) which adds/drops optical signals of an arbitrary wavelength at each node has been studied for being put into practical use. Adoption of this ROADM system makes it possible to switch optical paths by changing the wavelengths, in addition to expansion in the transmission capacity achieved by multiplexing the wavelengths. This results in drastic improvements in the versatility in use of optical networks. In this case, light sources corresponding to respective wavelengths are required for the optical communication network system.
A tunable laser device shown in FIG. 4 is known as a light source for a WDM transmission system (see Japanese Unexamined Patent Publication 2006-245346 (Patent Document 1), for example) FIG. 4A is a plan view showing a tunable laser device related to the present invention. FIG. 4B is a plan view which shows an enlarged view of a directional coupler shown in FIG. 4A. Explanations will be provided hereinafter by referring to the drawings.
A tunable laser device 70 includes: an optical filter 72 formed on a PLC 71; an SOA 73 which supplies light to the optical filter 72; a high-reflective coating 74 which returns light transmitted through the optical filter 72 to the SOA 73 via the optical filter 72; and optical waveguides 75 and 76 which are formed on the PLC 71 to connect the SOA 73, the optical filter 72, and the high-reflective mirror 74. The optical filter 72 is configured with ring resonators 77, 78 having different optical lengths from each other, and an optical waveguide 79 which connects the ring resonators 77, 78. Thin-film heaters 80a, 80b, 81a, and 81b for changing the phase of the light transmitted through the ring resonators 77, 78 are provided on the ring resonators 77, 78. To change the phase of the light is to change the wavelength of the light.
The optical waveguide 75 and the ring resonator 77 are optically coupled via a directional coupler 91, and the ring resonator 77 and the optical waveguide 79 are optically coupled via a directional coupler 92. Further, the optical waveguide 79 and the ring resonator 78 are optically coupled via a directional coupler 93, and the ring resonator 78 and the optical waveguide 76 are optically coupled via a directional coupler 94.
While FIG. 4B shows only the directional coupler 91, the other directional couplers 92-94 have the same structure as well. The directional coupler 91 is configured with two optical waveguides 95 and 96, and it exhibits a property that is determined depending on a gap g provided therebetween and a coupling length l. The optical waveguide 95 is a part of the optical waveguide 75, and the optical waveguide 96 is a part of the ring resonator 77.
As described, the tunable laser device 70 is in a structure in which the PLC 71 forms the optical filter 72, and the SOA 73 is directly mounted on the PLC 71. The two ring resonators 77 and 78 formed on the PLC 71 have a slightly different circumference from each other. Vernier effect occurs due to the difference in the circumferences, so that output light 82 of a wide tunable wavelength range can be obtained through controlling on/off of the thin-film heaters 80a, etc.
However, there are following issues in the tunable laser device 70 shown in FIG. 4.
When the manufacturing condition for the PLC 71 fluctuates, the property of the directional couplers 91-94 fluctuates as well. Accordingly, the wavelength transmittance property of the optical filter 72 greatly varies, so that the yield of the tunable laser device 70 is deteriorated greatly. This is because the directional couplers 91-94 are formed with the optical waveguides 95 and 96 arranged in parallel with the extremely narrow gap g of about 1.5 μm, so that the coupling property of the directional couplers 91-94 becomes greatly changed even with a change of 0.1 μm in the gap g.